The objective of this research is to study the role of aging on the differentiation of sensory neurons within the central nervous system. The experiments will focus on the affect of removal of the primary afferent fibers on the formation or maintenance of the normal, morphological features of differentiation of the target neurons. Particular attention will be paid to the age of the animal at the time of the surgical manipulations. The neurons selected for study are the principal cells of the chick tangential nucleus, a component of the lateral vestibular complex in avian species. The vestibular nerve will be deafferented before fiber ingrowth into the brain in embryos or lesioned in adults. This system has previously been studied at the light microscope level and is the basis to continue the investigation at the ultrastructural level. Furthermore, replacement of a developing ear with a developing eye will be studied with the light microscope using the rapid Golgi and reduced silver preparations. Evidence of synaptogenesis between principal cells and retinal axons will be pursued with the electron microscope.